1. Field of the Invention
The present invention relates to a semiconductor package, and more particularly to an outer lead structure of a resin sealed semiconductor package.
2. Description of the Related Art
Along with the downsizing trend in electronic equipment utilizing semiconductor devices such as a portable telephone set, notebook personal computer and the like, fabrication of semiconductor packages in small size is being demanded for the purpose of improving the packaging density of the semiconductor devices.
One type of semiconductor packages responding to the demand is that of thin and small outline package (TSOP) which makes it possible to obtain a flat and thin product. A perspective view of a TSOP is shown in FIG. 11, in which 11a is an outer lead and 11b is a resin sealed body made of an epoxy resin or the like.
Such a semiconductor package is typically fabricated according to the following procedure. First, a semiconductor element is bonded to a semiconductor element mount part of a lead frame, then electrodes (pads) formed on the semiconductor element are connected to an inner lead part of the lead frame with bonding wires such as gold wires. Next, these parts are sealed in a resin sealing body made of an epoxy resin or the like. The resin sealed semiconductor device is then subjected to a solder plating or the like on the surface of the outer leads in order to facilitate mounting by soldering onto a mount board such as a printed wiring board. Next, after cutting off unnecessary parts of the lead frame, the outer leads are worked into a gull wing shape regulated by predetermined dimensions. Finally, markings are given on the surface of the resin sealed body, completing the assembly process of the semiconductor device.
The semiconductor device fabricated according to the above processes is mounted on a mount board such as a printed wiring board. The outer leads 11a led out of the resin sealed body 11b of the lead frame are connected to the inner leads (not shown) within the resin sealed body 11b. Consequently, the electrode formed on the semiconductor element is connectable to a wiring on the mount board via a bonding wire, inner lead, and outer lead 11a.
FIG. 12 is a partial enlarged sectional view of the region from the peripheral part to the outer lead of the resin sealed body of the TSOP shown in FIG. 11. In FIG. 12, the length and the angle of the outer lead are drawn in proportion to the length and the angle of the outer lead of an actual semiconductor package, the thickness of the outer lead is drawn emphatically considering ease in viewing the drawing.
The dimensions and the angle of the outer lead 11a led out of the resin sealed body 11b are regulated to predetermined values according to the standards of the following five items.
In FIG. 12, 11c is the item regulating the horizontal distance from the edge surface of the resin sealed body 11b to the terminating part of the outer lead worked into the gull wing shape.
Item 11d regulates the vertical distance from the base plane of the outer lead 11a worked into the gull wing shape to the base plane of the resin sealed body 11b.
Item 11e regulates the planarity (angle .alpha.) of the base plane of the outer lead 11a worked into the gull wing shape.
Item 11f regulates the base plane length from the second inflection point of the outer lead 11a worked into the gull wing shape to the terminating part of the outer lead.
Item 11g regulates the thickness of the outer lead 11a.
Finally, item 11h regulates the height of the upper surface of the resin sealed body 11b as measured from the base plane of the outer lead, although it does not regulate directly the outer shape and the dimensions of the outer lead 11a.
In a miniaturized semiconductor device such as a TSOP, the outer leads that are worked into a gull wing shape are also formed to have a very short length. Problems entailing short formation of the outer leads will be described below.
In a semiconductor device, the semiconductor element itself sealed within the resin sealed body 11b generates heat as a result of its electrical activities. A part of the heat generated by the semiconductor element is dissipated directly into the surroundings of the semiconductor device via the resin sealed body 11b which is sealing the semiconductor element. Another part of the generated heat is dissipated into the surrounding atmosphere and to the mount board on which the semiconductor device is mounted, from the electrodes formed on the semiconductor element via the bonding wires, inner leads and the outer leads led out of the resin sealed body 11b. The mount board is heated by the heat generated by the semiconductor device and other electronic components, and is also heated by the electric currents flowing in the metallic wiring layer patterned on the mount board itself.
Accompanying such heat generation and heat dissipation in the semiconductor device and the mount board, the resin sealed body, the outer leads, and the mount board undergo elongation and contraction though they are slight. The amounts of elongation and contraction differ respectively depending upon the difference in the linear coefficient of thermal expansion of the resin sealed body, the outer lead, and the mount board as well as the temperature of respective parts.
Under these circumstances, when the resin sealed body, the outer leads, and the mount board repeat elongation and contraction accompanying heat generation and heat dissipation in the semiconductor device, the bonding strength of the solder connecting the semiconductor device to the mount board is deteriorated, and in the worst situation, a failure will occur in which some of the outer leads of the semiconductor device peel off the mount board.
Besides the elongation and contraction caused by the generation of heat in the semiconductor device itself, the resin sealed body, the outer leads, and the mount board undergoes, though slight, elongation and contraction due to temperature changes in the environment in which the semiconductor device and the mount board that has the semiconductor device on it are installed or otherwise left standing. Similar to the description in the above, as the effect of the temperature changes in the environment alone, or as the synergic effect of the temperature changes and of elongation and contraction caused by heat generation and heat dissipation in the semiconductor device itself, the bonding strength of soldering of the outer leads is deteriorated, and in the worst situation, a failure will occur in which the outer leads peel off the mount board.
In a conventional flat package type semiconductor device having outer leads 11a worked into a gull wing shape, the values for items 11c to 11h which are regulating the height of the semiconductor device and the external dimensions of the outer lead 11a are set at low levels in order to raise the surface mountability of the device. In particular, since the lead length (11x) from a first inflection point on the semiconductor device side to a second inflection point is set to be small (11x&lt;11f), the outer leads 11a are unable to sufficiently fulfill the function as a buffer material against the difference in the elongation/contraction generated respectively in the resin sealed body, the outer leads, and the mount board caused by heat generation/heat dissipation of the semiconductor device itself and/or by the temperature changes in the environment in which the semiconductor device and the mount board on which the semiconductor device is mounted are installed or otherwise left standing. As a result, the bonding strength of the solder is deteriorated, and in the worst situation, the problem of peeling of the outer leads of the semiconductor device from the mount board becomes more serious.
On the other hand, in Publication of Unexamined Patent Applications No. Hei 2-14558 there is proposed, as shown in FIGS. 13 and 14, to bend a free end part 22a of an outer lead 22, worked into a gull wing shape, in the direction to be away from a printed wiring board 24 on which a semiconductor integrated device 21 is mounted. By working the outer leads 22 in this manner, it is said that a sufficient "wettability to solder 25" can be obtained at the free end parts 22a, and a sufficient soldering strength can be secured.
In the conventional example described in Publication of Unexamined Patent Applications No. Hei 2-14558, although the strength of the soldering itself is enhanced by bending the terminating parts of the outer leads worked into a gull wing shape, the fact remains unchanged also in the example shown in FIG. 12 that the length (x) of the outer lead from the first inflection point to the second inflection point is short (x&lt;f). Consequently, the outer leads do not function sufficiently well as a buffer material against elongation and contraction, the bonding strength of the solder which is connecting the semiconductor device to the mount board is deteriorated as a result of repeated elongation and contraction of the various parts, and in the worst situation, the problem that the outer leads of the semiconductor device peel off the mount board still remains unresolved.